Emergent insights from the synthesis of conceptual frameworks for biological invasions

A general understanding of biological invasions will provide insights into fundamental ecological and evolutionary problems and contribute to more efficient and effective prediction, prevention and control of invasions. We review recent papers that have proposed conceptual frameworks for invasion biology. These papers offer important advances and signal a maturation of the field, but a broad synthesis is still lacking. Conceptual frameworks for invasion do not require invocation of unique concepts, but rather should reflect the unifying principles of ecology and evolutionary biology. A conceptual framework should incorporate multicausality, include interactions between causal factors and account for lags between various stages. We emphasize the centrality of demography in invasions, and distinguish between explaining three of the most important characteristics by which we recognize invasions: rapid local population increase, monocultures or community dominance, and range expansion. As a contribution towards developing a conceptual synthesis of invasions based on these criteria, we outline a framework that explicitly incorporates consideration of the fundamental ecological and evolutionary processes involved. The development of a more inclusive and mechanistic conceptual framework for invasion should facilitate quantitative and testable evaluation of causal factors, and can potentially lead to a better understanding of the biology of invasions.     

Quantitation of Malaria Parasite-Erythrocyte Cell-Cell Interactions Using Optical Tweezers

Erythrocyte invasion by Plasmodium falciparum merozoites is an essential step for parasite survival and hence the pathogenesis of malaria. Invasion has been studied intensively, but our cellular understanding has been limited by the fact that it occurs very rapidly: invasion is generally complete within 1 min, and shortly thereafter the merozoites, at least in in vitro culture, lose their invasive capacity. The rapid nature of the process, and hence the narrow time window in which measurements can be taken, have limited the tools available to quantitate invasion. Here we employ optical tweezers to study individual invasion events for what we believe is the first time, showing that newly released P. falciparum merozoites, delivered via optical tweezers to a target erythrocyte, retain their ability to invade. Even spent merozoites, which had lost the ability to invade, retain the ability to adhere to erythrocytes, and furthermore can still induce transient local membrane deformations in the erythrocyte membrane. We use this technology to measure the strength of the adhesive force between merozoites and erythrocytes, and to probe the cellular mode of action of known invasion inhibitory treatments. These data add to our understanding of the erythrocyte-merozoite interactions that occur during invasion, and demonstrate the power of optical tweezers technologies in unraveling the blood-stage biology of malaria.     

Evolutionary quantitative genetics of nonlinear developmental systems

In quantitative genetics, the effects of developmental relationships among traits on microevolution are generally represented by the contribution of pleiotropy to additive genetic covariances. Pleiotropic additive genetic covariances arise only from the average effects of alleles on multiple traits, and therefore the evolutionary importance of nonlinearities in development is generally neglected in quantitative genetic views on evolution. However, nonlinearities in relationships among traits at the level of whole organisms are undeniably important to biology in general, and therefore critical to understanding evolution. I outline a system for characterizing key quantitative parameters in nonlinear developmental systems, which yields expressions for quantities such as trait means and phenotypic and genetic covariance matrices. I then develop a system for quantitative prediction of evolution in nonlinear developmental systems. I apply the system to generating a new hypothesis for why direct stabilizing selection is rarely observed. Other uses will include separation of purely correlative from direct and indirect causal effects in studying mechanisms of selection, generation of predictions of medium‐term evolutionary trajectories rather than immediate predictions of evolutionary change over single generation time‐steps, and the development of efficient and biologically motivated models for separating additive from epistatic genetic variances and covariances.     

Quantitation of Malaria Parasite-Erythrocyte Cell-Cell Interactions Using Optical Tweezers

Erythrocyte invasion by Plasmodium falciparum merozoites is an essential step for parasite survival and hence the pathogenesis of malaria. Invasion has been studied intensively, but our cellular understanding has been limited by the fact that it occurs very rapidly: invasion is generally complete within 1 min, and shortly thereafter the merozoites, at least in in vitro culture, lose their invasive capacity. The rapid nature of the process, and hence the narrow time window in which measurements can be taken, have limited the tools available to quantitate invasion. Here we employ optical tweezers to study individual invasion events for what we believe is the first time, showing that newly released P. falciparum merozoites, delivered via optical tweezers to a target erythrocyte, retain their ability to invade. Even spent merozoites, which had lost the ability to invade, retain the ability to adhere to erythrocytes, and furthermore can still induce transient local membrane deformations in the erythrocyte membrane. We use this technology to measure the strength of the adhesive force between merozoites and erythrocytes, and to probe the cellular mode of action of known invasion inhibitory treatments. These data add to our understanding of the erythrocyte-merozoite interactions that occur during invasion, and demonstrate the power of optical tweezers technologies in unraveling the blood-stage biology of malaria.     

Is it Good to Make Happy People?

Would it be good, other things being equal, for additional people to exist whose lives would be worth living? I examine and reject several arguments for the answer that it would not be good; then I offer opposing arguments that I believe are more successful. Thus, I agree with utilitarians who say that it is better for there to be more happy people. Next I argue for the stronger claim that the happiness of potential people is as important as that of adults. Potential quality of life, then, matters in a host of bioethical issues: abortion, commercial surrogacy, the treatment of defective newborns, and so on. What is the practical upshot of all this? I reject the idea that we must do whatever is necessary to prolong life worth living. But I also reject the view that the side-effects of overpopulation always outweigh the value of realizing potential happiness. So I advocate a middle position, which I do not identify precisely. Even from this middle position, however, potential happiness is more important than is commonly assumed in bioethics.     

基于功能性状多维性理解外来物种入侵的达尔文归化难题

达尔文归化难题是进行外来物种入侵预测的重要理论依据,然而,达尔文归化假说和预适应假说却预测了2种截然不同的结果。事实上,达尔文归化难题争论的焦点是物种间的差异性还是相似性促进了外来物种的成功入侵,究其原因可能是忽略了功能性状的多维性。所谓功能性状的多维性,就是不同的功能性状可能代表着不同的生态功能轴,外来物种的入侵是多个维度上不同生态过程的综合结果。本研究以现代物种共存理论为基础,构建了一个基于环境过滤和适合度差异2个维度的入侵预测模型框架,不同维度对应着不同的功能性状以及不同的种间相似性关系。该预测模型表明,在环境过滤维度上与本地物种性状趋同,同时,在适合度维度上与本地物种性状趋异的外来物种是潜在的入侵物种,而其危害程度主要取决于本地群落的构建过程。该模型框架可为外来物种入侵预警提供理论依据,也可为生物多样性保护、外来物种的防治与管理等提供实践指导。     

Review: protein secondary structure prediction continues to rise

Methods predicting protein secondary structure improved substantially in the 1990s through the use of evolutionary information taken from the divergence of proteins in the same structural family. Recently, the evolutionary information resulting from improved searches and larger databases has again boosted prediction accuracy by more than four percentage points to its current height of around 76% of all residues predicted correctly in one of the three states, helix, strand, and other. The past year also brought successful new concepts to the field. These new methods may be particularly interesting in light of the improvements achieved through simple combining of existing methods. Divergent evolutionary profiles contain enough information not only to substantially improve prediction accuracy, but also to correctly predict long stretches of identical residues observed in alternative secondary structure states depending on nonlocal conditions. An example is a method automatically identifying structural switches and thus finding a remarkable connection between predicted secondary structure and aspects of function. Secondary structure predictions are increasingly becoming the work horse for numerous methods aimed at predicting protein structure and function. Is the recent increase in accuracy significant enough to make predictions even more useful? Because the recent improvement yields a better prediction of segments, and in particular of beta strands, I believe the answer is affirmative. What is the limit of prediction accuracy? We shall see.     

Comparison of the predicted and observed secondary structure of T4 phage lysozyme.

Predictions of the secondary structure of T4 phage lysozyme, made by a number of investigators on the basis of the amino acid sequence, are compared with the structure of the protein determined experimentally by X-ray crystallography. Within the amino terminal half of the molecule the locations of helices predicted by a number of methods agree moderately well with the observed structure, however within the carboxyl half of the molecule the overall agreement is poor. For eleven different helix predictions, the coefficients giving the correlation between prediction and observation range from 0.14 to 0.42. The accuracy of the predictions for both beta-sheet regions and for turns are generally lower than for the helices, and in a number of instances the agreement between prediction and observation is no better than would be expected for a random selection of residues. The structural predictions for T4 phage lysozyme are much less successful than was the case for adenylate kinase (Schulz et al. (1974) Nature 250, 140-142). No one method of prediction is clearly superior to all others, and although empirical predictions based on larger numbers of known protein structure tend to be more accurate than those based on a limited sample, the improvement in accuracy is not dramatic, suggesting that the accuracy of current empirical predictive methods will not be substantially increased simply by the inclusion of more data from additional protein structure determinations.     

Propagule pressure as a driver of establishment success in deliberately introduced exotic species: fact or artefact?

A central paradigm in invasion biology is that more releases of higher numbers of individuals increase the likelihood that an exotic population successfully establishes and persists. Recently, however, it has been suggested that, in cases where the data are sourced from historical records of purposefully released species, the direction of causality is reversed, and that initial success leads to higher numbers being released. Here, we explore the implications of this alternative hypothesis, and derive six a priori predictions from it. We test these predictions using data on Acclimatization Society introductions of passerine bird species to New Zealand, which have previously been used to support both hypotheses for the direction of causality. All our predictions are falsified. This study reaffirms that the conventional paradigm in invasion biology is indeed the correct one for New Zealand passerine bird introductions, for which numbers released determine establishment success. Our predictions are not restricted to this fauna, however, and we keenly anticipate their application to other suitable datasets.     

Causation in the sciences: An inferentialist account

I present an alternative account of causation in the biomedical and social sciences according to which the meaning of causal claims is given by their inferential relations to other claims. Specifically, I will argue that causal claims are (typically) inferentially related to certain evidential claims as well as claims about explanation, prediction, intervention and responsibility. I explain in some detail what it means for a claim to be inferentially related to another and finally derive some implication of the proposed account for the epistemology, semantics and metaphysics of causation.     

Biological invasions at the gene level

Despite several recent contributions of population and evolutionary biology to the rapidly developing field of invasion biology, integration is far from perfect. I argue here that invasion and native status are sometimes best discussed at the level of the gene rather than at the level of the species. This, and the need to consider both natural (e.g. postglacial) and human‐induced invasions, suggests that a more integrative view of invasion biology is required.     

Omnivory and stability in freshwater habitats: Does theory match reality?

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Historical factors mediate the effects of interspecific competition

Biotic interactions, such as interspecific competition, are potentially important in determining whether introduced species succeed or fail to establish wild populations. Such effects may be difficult to detect, however, because the outcome of interspecific competition may depend on historical and largely unpredictable circumstances such as the timing of introductions and the number of individuals of each species introduced. I used a stochastic birth-death model to explore the effects of interspecific competition, the timing of introductions and the numbers of individuals of each species introduced, on invasion success in a two-species competitive system. I then compared the model predictions with actual data on establishment outcomes for passerine birds introduced to New Zealand, for which we have data on the timing of introductions, the size of release populations, and a measure of the strength of per capita competition (the degree of morphological similarity among species). The model and data agree well, suggesting that interspecific competition was an important determinant of invasion success in this assemblage, but that the outcome of competition depended critically on circumstances such as the timing of introductions and number of individuals released. Hence, while there is a deterministic component to invasion success in this assemblage (morphologically similar species are less likely to establish), historical circumstances played a critical role in mediating the outcomes.     

Biodiversity and invasibility in grassland microcosms

In the years since Charles Elton proposed that more diverse communities should be less susceptible to invasion by exotic species, empirical studies have both supported and refuted Elton's hypothesis. Here, I use grassland community microcosms to test the effect of functional diversity on the success of an invasive annual weed (Centaurea solstitialis L.). I found that high functional diversity reduced the success of Centaurea by reducing resource availability. An equally important, but unstudied, question is whether diversity can buffer a community against the impacts of invasive species. In this experiment, although species diversity (independent of functional diversity) did not affect the success of the invader, the invader suppressed growth of species-poor communities more strongly. Invasion of Centaurea also increased summer evapotranspiration in species-poor communities. These results suggest that loss of species diversity alone does not affect community invasibility, but that communities with fewer species may be more likely to decline as a consequence of invasion.     

The role of competition and introduction effort in the success of passeriform birds introduced to New Zealand

Abstract The finding that passeriform birds introduced to the islands of Hawaii and Saint Helena were more likely to successfully invade when fewer other introduced species were present has been interpreted as strong support for the hypothesis that interspecific competition influences invasion success. I tested whether invasions were more likely to succeed when fewer species were present using the records of passeriform birds introduced to four acclimatization districts in New Zealand. I also tested whether introduction effort, measured as the number of introductions and the total number of birds released, could predict invasion outcomes, a result previously established for all birds introduced to New Zealand. I found patterns consistent with both competition and introduction effort as explanations for invasion success. However, data supporting the two explanations were confounded such that the greater success of invaders arriving when fewer other species were present could have been due to a causal relationship between invasion success and introduction effort. Hence, without data on introduction effort, previous studies may have overestimated the degree to which the number of potential competitors could independently explain invasion outcomes and may therefore have overstated the importance of competition in structuring introduced avian assemblages. Furthermore, I suggest that a second pattern in avian invasion success previously attributed to competition, the morphological overdispersion of successful invaders, could also arise as an artifact of variation in introduction effort.     

Experimental island invasion of house mice

The ability of invasive species to recurrently establish populations from small numbers of founders, while threatened species struggle at the same low population sizes, is a paradox in conservation biology. Little is known about the mechanisms contributing to the post-arrival success of low density invasive populations as most invasive species research focuses on established, high density populations. Experimental studies are powerful, but generally limited to laboratory or invertebrate experiments. Here, we experimentally demonstrate that vertebrate mammal invasion from a very small (n = 2) number of founders follows relatively simple deterministic predictions. An intentional island invasion of introduced house mice (Mus musculus Linnaeus) from one founding pair closely tracked the density dependent logistic growth curve and reached the seasonal carrying capacity of a previously extant population in only 5 months. Carrying capacity reflected both density dependent and independent processes. In contrast to the previously incumbent population, the invading population retained a marked genetic signal of its recent founder event, but the populations were otherwise demographically indistinguishable. Stochastic events such as individual variability, supplemental immigration and ecological release, but not Allee effects, played important roles during colonisation, but following establishment dynamics rapidly became deterministic, with little demographic impact of reduced genetic diversity. The small population paradigm appears to have little influence on the population dynamics of highly successful invasive species.     

Ernst Mayr,the tree of life,and philosophy of biology

Ernst Mayr’s influence on philosophy of biology has given the field a particular perspective on evolution, phylogeny and life in general. Using debates about the tree of life as a guide, I show how Mayrian evolutionary biology excludes numerous forms of life and many important evolutionary processes. Hybridization and lateral gene transfer are two of these processes, and they occur frequently, with important outcomes in all domains of life. Eukaryotes appear to have a more tree-like history because successful lateral events tend to occur among more closely related species, or at a lower frequency, than in prokaryotes, but this is a difference of degree rather than kind. Although the tree of life is especially problematic as a representation of the evolutionary history of prokaryotes, it can function more generally as an illustration of the limitations of a standard evolutionary perspective. Moreover, for philosophers, questions about the tree of life can be applied to the Mayrian inheritance in philosophy of biology. These questions make clear that the dichotomy of life Mayr suggested is based on too narrow a perspective. An alternative to this dichotomy is a multidimensional continuum in which different strategies of genetic exchange bestow greater adaptiveness and evolvability on prokaryotes and eukaryotes.     

In vitro cell culture methods for investigating Campylobacter invasion mechanisms

Studying the mechanisms of Campylobacter pathogenesis is complicated by the lack of simple animal models that mimic the disease seen in humans. In vitro cell culture methods provide a useful alternative to investigate the interactions between Campylobacter and the host epithelium that occur during infection. In the genomics era there is an increasing use of in vitro cell culture techniques to interrogate the potential role of different genes in pathogenesis. The aim of this review was to discuss the suitability and limitations of the various experimental approaches that might be adopted. We review current knowledge concerning the influence of cell-specific as well as bacterial factors required for Campylobacter invasion such as flagella and secreted proteins. The involvement and effects of phase variation on the results of invasion studies in cell culture emphasise the need to verify observed strain variations. We present the use of a mathematical Invasion Success Model to analyse Campylobacter invasion and show that it can be used to derive three strain dependent characteristics Imax, k, and I0. Even by combining data from independent experiments the Invasion Success Model can be used to statistically compare Campylobacter strains for their invasion of epithelial cells. Recommendations are given for the adoption of standard assay parameters and analytical methods such as the Invasion Success Model in order to facilitate comparison of data generated in different laboratories.     

Species Invasiveness in Biological Invasions: A Modelling Approach

The study of invasiveness, the traits that enable a species to invade a habitat, and invasibility, the habitat characteristics that determine its susceptibility to the establishment and spread of an invasive species, provide a useful conceptual framework to formulate the biological invasion problem in a modelling context. Another important aspect is the complex interaction emerging among the invader species, the noninvader species already present in the habitat, and the habitat itself. Following a modelling approach to the biological invasion problem, we present a spatially explicit cellular automaton model (Interacting Multiple Cellular Automata (IMCA)). We use field parameters from the invader Gleditsia triacanthos and the native Lithraea ternifolia in montane forests of central Argentina as a case study to compare outputs and performance of different models. We use field parameters from another invader, Ligustrum lucidum, and the native Fagara coco from the same system to run the cellular automaton model. We compare model predictions with invasion values from aerial photographs. We discuss in detail the importance of factors affecting species invasiveness, and give some insights into habitat invasibility and the role of interactions between them. Finally, we discuss the relevance of mathematical modelling for studying and predicting biological invasions. The IMCA model provided a suitable context for integrating invasiveness, invasibility, and the interactions. In the invasion system studied, the presence of an invader's juvenile bank not only accelerated the rate of invasion but was essential to ensure invasion. Using the IMCA model, we were able to determine that not only adult survival but particularly longevity of the native species influenced the spread velocity of the invader, at least when a juvenile bank is present. Other factors determining velocity of invasion detected by the IMCA model were seed dispersal distance and age of reproductive maturity. We derived relationships between species' adult survival, fecundity and longevity of both theoretical and applied relevance for biological invasions. Invasion velocities calculated from the aerial photographs agreed well with predictions of the IMCA model.